* Israel mulls calling up more reserves as fighting drags on
* Says accepts possible temporary humanitarian truce
* Abbas discusses truce plan despite Hamas's rejection
* Gaza death toll hits 215 after more air strikes
* Israel warns areas with 100,000 population to evacuate
((Adds possible temporary humanitarian truce, more air strikes)
By Nidal al-Mughrabi and Allyn Fisher-Ilan
GAZA/JERUSALEM, July 16 (Reuters) - Israeli shelling killed four boys on a Gaza beach on Wednesday, a local health official said, and Palestinian militants fired more than 100 rockets into Israel after a failed Egyptian attempt to halt more than a week of warfare.
Israel later on Wednesday agreed to a proposed six-hour cessation of hostilities for humanitarian reasons, a senior Israeli official said, adding that it had not yet been decided when the lull would take place.
The Islamist Hamas, which rules Gaza, had no immediate comment. The appeal came from the United Nations, the Israeli official said, shortly after Hamas rejected an Egyptian-proposed ceasefire to end the nine-day war.
After the beach shelling, six other Palestinians including four members of one family, among them a woman, 70, and two children aged 4 and 6, were killed in two Israeli air strikes in the southern Gaza city of Khan Younis, medics there said.
Israel urged the evacuation of several districts in the Gaza Strip where more than 100,000 people live, threatening a ground thrust into the densely populated coastal territory to try to stem the rocket attacks.
An Israeli official said the defence minister asked Prime Minister Benjamin Netanyahu's security cabinet to authorise the mobilisation of another 8,000 reserve troops. The military has said that around 30,000 reservists have been called up since the Israeli offensive began a week ago.
Netanyahu told mayors of rocket-struck towns: “We will continue to conduct this campaign until it goal is achieved. We will use as much force as necessary to restore quiet to Israel's residents.”
On the diplomatic front, the Islamist Hamas political leadership of Gaza formally rejected Cairo's ceasefire plan a day after its armed wing spurned it. Militants kept up rocket salvoes at Israel, which held its fire for six hours on Tuesday.
But despite the collapse of Egypt's truce efforts, there were signs of efforts still under way to nail down a ceasefire.
Palestinian President Mahmoud Abbas discussed Egypt's plan in Cairo on Wednesday with senior Hamas official Moussa Abu Marzouk, Egyptian and Palestinian news agencies said. The Palestinian agency WAFA said they held "in-depth" talks.
U.S. SAYS DOING UTMOST TO GET TRUCE
In Washington, Secretary of State John Kerry said the United States was doing "everything in our power" to achieve a truce.
Ashraf al-Qidra of the Gaza Health Ministry said shelling from an Israeli gunboat off Gaza's Mediterranean coast killed four boys - two aged 10 and the others 9 and 11 - from one family and critically wounded another youngster on the beach.
The Israeli military said the reported civilian casualties were unintended and "tragic" and it was investigating what happened. "Based on preliminary results, the target of this strike was Hamas terrorist operatives," it said in a statement.
Netanyahu says Israel's armed forces try to avoid civilian casualties but that militant rocket crews consciously put non-combatants at risk by operating in crowded residential areas.
Ahmed Abu Hassera, who witnessed the incident at the shore, told Reuters: "The kids were playing on the beach. They were all ... under the age of 15."
Israeli shelling has frequently targeted Gaza beaches, which are suspected staging areas for militants.
"When the first shell hit land, they ran away but another shell hit them all," said Abu Hassera, whose shirt was stained with blood. "It looked as if the shells were chasing them."
Reacting to the incident, Hamas spokesman Sami Abu Zuhri told reporters in Gaza: "These crimes will not succeed to break our will. We will continue the confrontation and resistance and we promise (Israel) will pay the price for all these crimes."
Earlier, Israeli attacks in the Gaza Strip killed at least eight Palestinians, five of them civilians, and a six-year-old boy died of wounds sustained a few days ago, Gaza medics said.
The two air strikes later in Khan Younis and the death of another Palestinian from wounds sustained in an earlier attack raised the death toll in Gaza to 215.
Gaza health officials say most of the Palestinian dead from in the worst flare-up of violence with Israel in two years have been civilians. Gaza's Al-Mezan Center for Human rights said 259 houses had been demolished by Israeli air strikes and 1,034 damaged along with 34 mosques and four hospitals.
The rocket volleys from Gaza have made a race to shelters a daily routine for hundreds of thousands in the Jewish state. One Israeli has been killed in the rocket fire, most of whose projectiles have crashed on open ground or been intercepted by Israel's Iron Dome anti-missile shield.
The military said Iron Dome shot down 29 of 123 rockets launched at Israel on Wednesday, while the others struck without causing casualties. One salvo, at coastal Ashkelon, forced visiting Norwegian Foreign Minister Borge Brende into a shelter.
Authorised by Netanyahu's security cabinet to escalate the offensive, the military relayed warnings to inhabitants in northern Gaza with dropped leaflets and mass phone calls.
But in the Shejaia and Zeitoun districts, bastions of popular support for Hamas and the Islamic Jihad faction, there was no sign residents were heeding the Israeli call to leave.
"Failure to comply will endanger your lives and the lives of your family," said a recorded message received by residents of Shejaia and Zeitoun, which sprawl out to the sandy, barbed-wire border with Israel.
Maher Abu Saa'ed, a 45-year-old doctor in Zeitoun, said that with many areas of Gaza under attack, nowhere was safe and he would not leave despite a telephoned Israeli warning to get out.
"To ask hundreds of people to leave their houses and go to the centre of the city is insane, a sick joke," he said.
Announcing Hamas's formal rejection of the ceasefire plan, Abu Zuhri said the outcome of deliberations within the Islamist movement "was to reject the proposal and, therefore, Hamas informed Egypt last night it apologises for not accepting it."
Hamas leaders have said any Gaza ceasefire must include an end to Israel's blockade of the territory, recommitment to a truce reached in an eight-day war there in 2012 and the release of hundreds of its activists arrested in the occupied West Bank while Israel hunted for three abducted Jewish seminary students.
The three teens were later found dead, and a Palestinian youth was later murdered in what appeared a revenge attack by Israelis. Those killings led to the current bout of hostilities.
Hamas also wants Egypt to ease curbs at its Rafah crossing with Gaza, imposed after the toppling of Islamist President Mohamed Mursi in Cairo a year ago.
The truce proposal announced by Egypt's Foreign Ministry spoke only in general terms about opening Gaza's borders and made no mention of the Hamas men held by Israel.
Kerry said in Washington when asked what he was doing about the crisis in Gaza, one of the world's most cramped territories where casualties have spiralled: "Our concern is to have a legitimate ceasefire and to see if we can find a way to stop the conflict and killing so that we can get to the real issues that are underlying it. We’re doing everything in our power."
Another round of U.S.-brokered talks to settle the six-decade-old Israeli-Palestinian conflict collapsed in April.
(Additional reporting by Maayan Lubell and Dan Williams in Jerusalem, Noah Browning in Gaza and Stephen Kalin, Michael Georgy and Yasmine Saleh in Cairo; Editing by Jeffrey Heller and Mark Heinrich)